In general, a fuel cell includes a membrane-electrode assembly that includes a pair of electrodes (an anode electrode and a cathode electrode) that sandwich two sides of an electrolyte membrane therebetween, and a pair of fuel cell separators that sandwich two sides of the membrane-electrode assembly therebetween. The anode electrode has an anode-electrode catalyst layer and a diffusion layer. The cathode electrode has a cathode-electrode catalyst layer and a diffusion layer. At the time of power generation in the fuel cell, when hydrogen gas is used as an anode gas supplied to the anode electrode and oxygen gas is used as a cathode gas supplied to the cathode electrode, a reaction occurs that generates hydrogen ions and electrons on the anode electrode side. The hydrogen ions pass through the electrolyte membrane to the cathode electrode side, while the electrons arrive at the cathode electrode through an external circuit. On the cathode electrode side, the hydrogen ions, electrons, and oxygen gas react to generate moisture and discharge energy.
Normally, since the cathode electrode side of a fuel cell communicates with the atmosphere, air from the atmosphere enters into the cells of the fuel cell when power generation is stopped. In some cases, the air that had entered may move from the cathode electrode side to the anode electrode side through the electrolyte membrane. There are also fuel cells that supply oxygen gas to the anode electrode side to stop power generation in the fuel cell, and discharge hydrogen gas that is inside the anode electrode.
As described above, when an anode gas such as hydrogen gas is supplied to a fuel cell in a state in which air (oxygen gas) exists in the anode electrode, a state occurs at the anode electrode in which the anode gas and air are unevenly distributed. As a result, a local battery is formed at a portion where the anode gas is unevenly distributed, while at a portion where the air is unevenly distributed, the current flows in the opposite direction to the direction at the time of normal power generation. Consequently, the electric potential of the fuel cell may become an abnormal potential and corrode the cathode electrode, to thereby lower the power generation performance of the fuel cell.
For example, Patent Document 1 proposes a fuel cell system in which the pressure of an anode gas supplied when starting the fuel cell is set to a higher pressure than the pressure of an anode gas supplied during power generation in the fuel cell. According to the fuel cell system disclosed in Patent Document 1, because an anode gas is supplied at a high pressure to the anode electrode when starting the fuel cell, uneven distribution of air inside the anode electrode can be suppressed, thus inhibiting the occurrence of an abnormal electric potential.
However, according to the fuel cell system disclosed in Patent Document 1, because a high-pressure anode gas is supplied to the anode electrode, there is a large difference between the gas pressure at the anode electrode and the gas pressure at the cathode electrode, and therefore an electrolyte membrane that is sandwiched between the anode electrode and the cathode electrode may be damaged.
Further, for example, Patent Document 2 proposes a fuel cell system in which a pressure difference between the gas pressure of an anode electrode and the gas pressure of a cathode electrode is controlled to less than or equal to a predetermined value, although in this case the pressure difference is not caused by starting a fuel cell. According to the fuel cell system disclosed in Patent Document 2, since a pressure difference between gases at an anode electrode and a cathode electrode can be maintained at less than or equal to a predetermined value, damage to an electrolyte membrane can be suppressed.    Patent Document 1: Japanese Patent Laid-Open Publication No. 2004-139984    Patent Document 2: Japanese Patent Laid-Open Publication No. 2002-373682